Cassini Finds an Atmosphere on Saturn's Moon Enceladus

by SPACE.com Staff | March 16, 2005 03:28pm ET

This false-color, close-up look at Enceladus yields new insight into the different processes that have shaped the moon’s icy surface. Extending through the center of this image, a system of two-mile (three-kilometer) wide rifts and 12-mile (20-kilometer) wide lanes of grooved terrain separate two distinct geological provinces on Enceladus. To the right of the boundary is older, cratered terrain – a region peppered with craters of all sizes from six miles (10 kilometers) in diameter, down craters near the limit of resolution. ImageCredit: NASA/JPL/Space Science Institute

The
Cassini spacecraft has revealed that Saturn's icy
moon Enceladus has a significant atmosphere.

The
spacecraft unlocked the moon's secret during recent flybys, NASA's Jet
Propulsion Laboratory announced Wednesday. Scientists, using Cassini's magnetometer instrument for their studies, said the
source may be volcanism, geysers, or gases escaping from the surface or the
interior.

When
Cassini had its first encounter with Enceladus on Feb. 17 at an altitude of 725 miles (1,167 km),
the magnetometer instrument saw a striking signature in the magnetic field. On
March 9, Cassini approached to within 310 miles (500
km) of Enceladus' surface and obtained additional
evidence.

The
observations showed a bending of the magnetic field, with the magnetospheric plasma being slowed and deflected by the
moon. In addition, magnetic field oscillations were observed. These are caused
when electrically charged (or ionized) molecules interact with the magnetic
field by spiraling around the field line. This interaction creates
characteristic oscillations in the magnetic field at frequencies that can be
used to identify the molecule. The observations from the Enceladus
flybys are believed to be due to ionized water vapor.

"These
new results from Cassini may be the first evidence of
gases originating either from the surface or possibly from the interior of Enceladus," said Dr. Michele Dougherty, principal
investigator for the Cassini magnetometer and
professor at Imperial College in London.
In 1981, NASA's Voyager spacecraft flew by Enceladus
at a distance of 56,000 miles (90,000 km) without detecting an atmosphere. It's possible detection was beyond Voyager's capabilities,
or something may have changed since that flyby.

This
is the first time since Cassini arrived in orbit
around Saturn last summer that an atmosphere has been detected around a moon of
Saturn, other than its largest moon, Titan. Enceladus
is a relatively small moon. The amount of gravity it exerts is not enough to
hold an atmosphere very long. Therefore, at Enceladus,
a strong continuous source is required to maintain the atmosphere.

The
need for such a strong source leads scientists to consider eruptions, such as
volcanoes and geysers. If such eruptions are present, Enceladus
would join two other such active moons, Io at Jupiter and Triton at Neptune. "Enceladus could
be Saturn's more benign counterpart to Jupiter's dramatic Io," said Dr.
Fritz Neubauer, co-investigator for the Cassini magnetometer, and a professor at the University of Cologne
in Germany.

Since
the Voyager flyby, scientists have suspected that this moon is geologically
active and is the source of Saturn's icy E ring. Enceladus
is the most reflective object in the solar system, reflecting about 90 percent
of the sunlight that hits it. If Enceladus does have
ice volcanoes, the high reflectivity of the moon's surface might result from
continuous deposition of icy particles originating from the volcanoes.

Enceladus' diameter is
about 310 miles (500 km), which would fit in the state of Arizona. Despite its small size, Enceladus exhibits one of the most interesting surfaces of
all the icy satellites.